Process of bleaching pulp



Feb. 12, 1935. c. "r. HENDERSON PROCESS OF BLEACHING PULP Filed Sept.26, 1933 Patented Feb. 12, 1935 UNITED STATES "PATENT OFFICE 8Claims.

This invention relates to processes of bleaching pulp; and it comprisesa method wherein wood pulp and similar cellulose materials are submittedto a preliminary purification by the action thereon of an aqueous liquidcontaining HOCl and made by passing chlorin and water into contact withcalcium carbonate, the amount of this liquid, being such as to produce adark liquor by reaction with the coloring matter of the pulp, the pulpis then drained and washed and the washed pulp is finally bleached withalkaline hypochlorite solution in an ordinary manner; all as more fullyhereinafter set forth and as claimed.

The primary object attained in this invention is providing a process ofbleaching pulp in which the total time and the chlorin requirements forcomplete bleaching and the fiber loss due to bleaching are all reduced;loss of strength in the fiber being also obviated.

Another result is the provision of a process of bleaching pulp orsimilar cellulose material wherein the pulp is first subjected to thechemical action of a particular aqueous liquid containing HOCl which canbe made by bringing a solution of C12 in water in contact with CaCOa.The pulp is then washed to remove coloring matter dissolved by the E001treatment and the washed pulp then is bleached with hpyochloritesolution.

It has been ordinary practice to bleach pulp with directly appliedhypochlorite solutions, such as the hypochlorites of sodium and ofcalcium; bleaching being carried on till commercially sat isfactorybleached material is produced. Sometimes the whole bleaching is effectedin one stage of operation and sometimes in two or more, with washingbetween stages.

I have discovered that pulp can be bleached with greater ease, lessfiber loss and in most cases with a less total consumption of chlorin,it treated in the following manner:

The pulp is first subjected to the action of a reaction productresulting from the bringing of a solution of Ch in water in intimatecontact with CaCOa. This can be accomplished by passing the Ch solutionthrough a vessel containing subdivided CaCOs. Advantageously I introducethe Ch solution at the bottom of the vessel and allow it to pass upwardsthrough the CaCOa as this procedure reduces the tendency to pack andgives better results.

The action upon the pulp of this reaction product, which consists of asolution containing H001 and CO2 together with calcium chlorid andcalcium bicarbonate, eifects a dissolving of color.

The pulp remains in contact with the solution until substantially allthe "available chlorin of the solution has disappeared. The contactbetween pulp and treating solution is made as intimate as possible bythe use of agitating or stirring devices. When the available chlorin hasdisappeared the pulp isdrained andwashed. The liquors drawn 0!! in thedraining and washing are dark colored and carry much of. the originalcoloring matter in the pulp.

The washing may be eflected with either fresh water or with a solutionof calcium hydroxide, Ca (0H) 2, in water.

After the washing operation, the pulp is now bleached in the usualmanner with a bleaching liquor containing sodium or calciumhypochlorite. It has been found that the time and total amountofavailable chlorin required to fully bleach the pulp is greatlyreduced, the total time and total available chlorin requirements,including the chlorin required to make the E001 solution for thepreliminary treatment, being less than would be required if thebleaching were done with sodium or calcium hypochlorite alone, that is,without the preliminary HOCl treatment. It has also been found that pulpcontaining greater percentages of lignin and similar incrustingmaterials, and therefore being harder to bleach can be readily bleachedbecause these incrusting materials are made soluble in the first step ofthe treatment and washed out prior to the bleaching.

The described treatment of the pulp may be considered as a purificationpreliminary to the actual bleaching. The chemistry involved is stillobscure; but on treating the pulp with the proper quantity of a liquormade by passing chlorin wa ter upwardly through a tower full oflimestone chips, the treatment being continued until disappearance fromthe liquor of available chlo (HOCl), non-cellulose constituents of thepulp are brought into solution to form a dark liquid; the color rangingfrom an amber to that of a dark beer. The pulp itself is not bleachedand after washing, is usually somewhat darker than it was before thetreatment. The actions involved were at first thought to be oxidation,but both oxidation and chlorination of lignin are precluded, since bothinvolve the production of HCl. RC1 is not produced from the HOCl in thepresent operation in substantial amount. Not more than about 15 per centof the chlorin of the HOCl reappears as chlorin ion; that is, ashydrochloric acid or as a chlorid. The bulk of the HOCl disappears assuch; it apparently goes into organic combination, forming bodiessoluble in water, and perhaps of a chlorhydrin nature, which pass awaywith the washwater. Whatever the chemistry involved,

the HOCl, in this particular liquid and under these particularconditions, acts to solubilize noncellulose constituents of the pulp,rather than to oxidize or chlorinate them.

The liquid delivered from the top of the tower has C0: acidity; it hasthe acidity due to the presence of calciumbicarbonate and free CO1.

It is, however, actually basic in the sense that the contained calciumbicarbonate will react with HCl. The amount of calcium bicarbonate inthis tower liquor varies somewhat with circumstances, but the amountpresent has significance. The liquid as it enters the pulp treater hasCO: acidity only and is not corrosive to apparatus. This enables the useof cheap structural materials and cheapens the cost of installation. Theamount of 1101 developed in the operation is very small and thedilution, great. Insofar as calcium bicarbonate is present, it takescare of this 1101. I have found that it is often desirable to supplementthe calcium carbonate in the tower by additions of calcium carbonate tothe tower liquor as it passes through the pulp treater. This takes careof the minimal amount of HCl developed and also preserves the initialCO: acidity.

The amount of tower liquor 'used varies with the pulp; with the amountof non-cellulose (lignin) in the pulp. I do not use sufiicient of it inany case to produce any considerable bleaching of the pulp. The bestresults are obtained by using such an amount that the contained HOCljust disappears at a time when nearly all the noncellulose has beenrendered soluble and forms a dark liquor. This precludes any chemicalattack on the fiber by the R001; and also the bleaching thereof.

In the present invention it is the object to treat the pulp with thedescribed solution, HOC], in such an amount and at such a dilution thatcomplete exhaustion of available chlorin (HOCl) in the solution and theproduction of a dark liquor occur together. In this treatment the fiberitself remains unbleached; in fact, as stated, after the HOCl treatmentfollowed by draining and washing the sheet pulp is often darker than theoriginal raw pulp.

In the bleaching of most pulps by the present invention the totalconsumption of chlorin is kept below that required in prior processesgenerally. In many cases the total amount of chlorin used ranges betweenand -pounds of chlorin per ton pulp on an air dried basis. Of thisamount usually between 50 and 70 per cent is applied in the first stageas the available chlorin of hypochlorous acid and the remainder isapplied in the bleaching with calcium hypochlorite. With a pulpconsistency around 3 to 4 per cent, the hypochlorous acid solution addedin the first stage is or may be in a concentration between 10 and 15grams of available chlorin per liter. This concentration of availablechlorin is substantially greater than that applied in most priorchlorination processes. Nevertheless the acidity of the pulp magmaremains substantially less.

In the final or bleaching stage of the process a relatively smallquantity of chlorin as hypochlorite bleach liquor is required and thebleaching action is rapid. The preliminary treatment with HOCl of thelignin and similar incrusting and coloring matter and their removal whenthe pulp is washed lightens the load on the bleaching step. In this stepthe pulp may be in a substantially higher consistency than that in thefirst step. A consistency about 15 to 20 per cent is in most casespracticable and advantageous. In bleaching, either sodium hypochloriteor calcium hypochlorite may be'used; bleaching being in the usual mannerand with the usual apparatus.

The process is advantageously applied to the bleaching of most pulpsin'a continuous preliminary HOCl treatment and continuous draining andwashing combined with a discontinuous or batch method in the final orbleaching step. This combined continuous and batch method of operationpermits of adjusting the final bleaching to meet variations in the rawpulp. It has the advantage of permitting close control of the quantitlesof materials used in the preliminary treatment and the time of treatmentso as to maintain uniform conditions.

In the accompanying drawing I have shown in more or less diagrammaticform an organization of apparatus useful in carrying on the describedprocess.

In this showing the figure is a view in vertical section, partly inelevation, of apparatus adapted for the combined continuous and batchmethod of applying theprocess. The figure may be taken to represent aflow sheet of the process.

A tower 1 is packed with ordinary limestone in lump or chip form andchlorin and water are introduced near the bottom of the column through avalved inlet 2. From the top of the tower, through an outlet 3, asolution containing free hypochlo'rous acid, HOCl, together with calciumchlorid, calcium bicarbonate and CO: in solution is led to a conduit 4through which a pulp magma of the desired consistency is introduced intothe bottom of a treatment tank 5 provided with stirring or agitatingmeans of usual form and designated 6. The treatment tank is of suitablehorizontal length and cross sectional area to permit a desired rate ofprogression of the magma through the tank so as to adjust the time ofthe preliminary HOCl treatment; the pulp magma being, as shown,introduced near the bottom of the tank at one end and withdrawn fromnear the top of the tank at the other end through a valved outlet 7. Asshown, the outlet '7 is .provided with a valved inlet 8 for introducingwater into the eiiluent magma. The outlet delivers into a rliile-box 9with an open top and provided with a series of horizontal riilies 10over which the magma trickles in order to maintain its consistencyuniform and to settle out particles of dirt.

From the riille-box the magma runs through a baflled inlet 11 into ascreen filter of a usual type shown at 12. In the filter, pulp isseparated from the liquor and collected in a sheet upon the screencylinder 13 within which a vacuum is maintained by applying suctionthereto. A series of water jets 14 are shown and the sheet of pulp iswashed with water as the cylinder 13 revolves. A doctor 15 scrapes thesheet of pulp from the cylinder and the sheet pulp is conveyed as by asuitable belt conveyor 16 to be introduced into bleaching tanks 17 inseparate batches. The bleaching tanks may be of the usual type.Hypochlorite bleaching solution is introduced into the several bleachingtanks through valved inlets 18. From the bleaching tanks the magma ofbleached pulp and spent bleaching liquor is removed through outlets 19and run through conduit 20 to a place for further disposal. In theconduit water may be added to the pulp magma through a valved inlet pipe21.

In continuous operation of the preliminary HOCl treatment, pulp ofdesired consistency is fed to the treatment tank through conduit 4 withaddition of the treating solution in a desired proportion from tower l;the tower being so designed that the liquid flow through the tower givesa time of contact with the limestone sufiicient to dissolve asubstantial amount of the lime as bicarbonate. However, finely dividedcalcium carbonate may be added, if desired, to the pulp in conduit 4.

In the treatment tank the mixture of pulp magma and tower efiiuent isslowly stirred and the rate of feed of pulp and admixed tower efiluentcontrols the time of treatment; 20 to 30 minutes being usuallysuflicient. The pulp magma runs continuously from the treatment tank andover the rifiies where dirt may settle out. Thence the pulp runs to thefilter where it is continuously collected in sheets, washed and conveyedto one of the several bleachers 17. These are used in succession.Bleaching in separate batches permits variation of the bleaching timeand uniform control of the bleaching operation.

While the described process is applicable to bleaching of wood pulps andpaper pulps and cellulosic materials generally, particularlyadvantageous results have been obtained in bleaching pulps of the typeknown as hard bleaching such as hard bleaching sulfite and kraft woodpulps. Hard bleaching pulps have a strong fiber, but the strength offiber is apt to be lessened by the drastic bleaching required. In thepresent process, the preliminary treatment with hypochlorous acid in aweakly acid solution may be said to convert a hard bleaching pulp intoan easy bleaching pulp, with a reduction of the amount of bleach liquorused in the bleaching proper more than sufi'icient to compensate for thechlorin used in the preparation of the hypochlorous acid solution. Ithas been found that the strength of the fiber remains substantiallyhigher than when the pulp is bleached to the same commercial grade inordinary bleaching methods using hypochlorite alone. In one case, thebleached pulp produced by the present process when compared with thatproduced by ordinary hypochlorite bleaching showed an increase of 10points in the Mullen test, an increase of 60 per cent in fold test andof 40 per cent in tear test. The alpha cellulose content was increasedfrom 81.30 per cent to 85.15 per cent and the copper number was lower,being 4.41 instead of 4.88. The chlorin consumption was lowered aboutone-third. In other words, there was a better product and a lessconsumption of chlorin. The operation was also quicker. The apparatusrequired is simple and inexpensive.

In preparing the hypochlorous acid solution, instead of limestone I canuse marble chips, chalk or any other form of calcium carbonate. Dolomitechips are also applicable, the magnesium carbonate therein beingattacked by the chlorin water with CO2 evolution and formation ofanother alkaline earth bicarbbnate in addition to thecalciumbicarbonate. It being a matter of neutralizing HCl without alsoneutralizing HOCl, any carbonate of a normally alkaline base isapplicable, which is converted by chlorin water into a solublebicarbonate. The alkaline earth carbonates such as calcium and magnesiumcarbonates react readily with chlorin water to give a solutioncontaining free hypochlorous acid together with the correspondingchlorids and bicarbonates and with CO2 acidity due to the bicarbonateand to dissolve CO2 formed from the original carbonate.

This application is a continuation-in-part of application Serial No.390,376, filed Sept. 4, 1929.

What I claim is:-

1. In the bleaching of wood pulp and similar cellulose materials, theprocess which comprises passing chlorin and water into contact withfragmental limestone, treating wood pulp with the efiiuent liquor insuch amount that all available chlorin therein can be exhausted withoutbleaching the pulp, draining and washing the pulp and thereafterbleaching the pulp with a hypochlorite bleaching solution.

2. In the process of claim 1, adding solid calcium carbonate to the pulpduring treatment with said efiiuent liquor.

3. In bleaching wood pulp with a solution of an hypochlorite of analkaline or alkaline earth metal, an improvement which comprises thepreliminary steps of treating the pulp with a solution of freehypochlorous acid containing also calcium chlorid, calcium bicarbonateand dissolved CO2, said solution being limited in amount to thatsufficient to produce a dark liquor and insufficient to bleach the pulp,and of draining and washing the pulp prior to the bleaching.

4. A method of bleaching wood pulp and similar cellulose materials whichcomprises a preliminary step of treating pulp in a continuous flow withan aqueous solution containing HOCl together with chlorid andbicarbonate of an alkaline earth metal and dissolved CO2 in amountsrequired to give a dark liquor and unbleached pulp, an intermediate stepof continuously draining and washing the treated pulp and a subsequentstep of bleaching the washed pulp in batches with a hypochloritebleaching solution.

5. In the bleaching of wood pulp, the process which comprises preparinga solution containing free HOCl together with an alkaline earth chloridand bicarbonate and dissolved CO2, treating pulp with said solution inmerely the amount required to give a dark liquor and. an unbleachedfiber, washing out the dark liquor and thereafter bleaching the fiber.

6. In the process of claim 5, maintaining a C02 acidity throughout theHOCl treatment.

7. In the bleaching of wood pulp and similar cellulose materials, theprocess which comprises passing chlorin and water into contact withfragments of an alkaline earth carbonate to make a liquid efiluentcontaining HOCl, alkaline earth chlorid and bicarbonate and dissolvedCO2, treating the pulp with said efiiuent in an amount insuificient tobleach the pulp, draining and washing the pulp and thereafter bleachingthe pulp with. a hypochlorite bleaching solution.

' 8. In the process of claim 7, adding solid alkaline earth carbonate tothe pulp during said HOCl treatment.

CLARK T. HENDERSON.

